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CHAPTER-2

ENTITY RELATIONSHIPS

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ENTITY

An entity is a real-world item or concept that exists on its own. Theset of all possible values for an entity is the entity type.

In our example, a particular student (such as, Ram"), team,

lab section, or experiment is an entity.

The set of all possible values for an entity, such as all possiblestudents, is the entity type.

In an ER model, we diagram an entity type as a rectangle

containing the type name, such asstudent

(see Figure 1).

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ER Model.

This data model is based on real world thatconsists of basic objects called entities and ofrelationship among these objects.

Entities are described in a database by a set ofattributes.

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Attribute

An attribute of an entity is a particular property that describes the entity. The setof all possible values of an attribute is the attribute domain.

Each entity has attributes, or particular properties that describe the entity.

The set of all possible values of an attribute, such as integers from 0 to 100 forgrade is the attribute domain.

In an ER model, an attribute name appears in an oval that has a line to thecorresponding entity box, shown in figure 2.

ER diagram notation for an attribute domain (Student Grade) of an entity type

(student)

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A simple attribute is one component that is atomic. A composite attribute has

multiple components, each of which is atomic or composite.

A composite attribute, such as RAM VYAS", has multiplecomponents, suchas RAM" and "VYAS"; and each component is atomic or composite. We

illustrate this composite nature in the ER model by branching off the

component attributes, such as in Figure 3.

ER diagram notation for composite attribute domain,

name

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A derived attribute can be obtained from other attributes or related entities. For

example, the radius of a sphere can be determined from the circumference.

We request the derived attribute with a dotted oval and line, such as in Figure 5.

Figure 6. ER diagram notation for derived attribute, radius

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ER Modeling Symbols.

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Sub class Specialization.

As you are developing a class diagram, you might discover that one ormore attributes of a class are characteristics of only someindividuals ofthat class, but not of others. This probably indicates that you need todevelop a subclass of the basic class type.

We call the process of designing subclasses from top downspecialization; a class that represents a subset of another class typecan also be called a specialization of its parent class.

Example:we will model the graduate students at a university. Some are

employed by the university as teaching associates (TAs); some are employed as

research associates (RAs); some are not employed by the university at all.

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Sub class Specialization continues..

Two specialized classes of students: TAs and RAs. The UMLsymbol for subclass association is an open arrowhead thatpoints to the parent class.

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Generalization

The reverse of the specialization process

Several classes with common features are generalizedinto a superclass; original classes become itssubclasses

Example: CAR, TRUCK generalized into VEHICLE; bothCAR, TRUCK become subclasses of the superclassVEHICLE.

We can view {CAR, TRUCK} as a specialization ofVEHICLE

Alternatively, we can view VEHICLE as ageneralization of CAR and TRUCK

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Superclass Generalization

Bottom up design

Sometimes, instead of finding unique attributes in a single class type, you mightfind two or more classes that have many of the sameattributes. This probably

indicates that you need to develop a superclass of the classes with commonattributes

We call the process of designing subclasses from bottom up generalization; aclass or entity that represents a superset of other class types can also be called a

generalization of the child types.

Example.It seemed as if there were two class types:

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Superclass Generalization continues..

Actually, a few of the lots were identified by both address schemes. The common

attributes should go in a generalization or superclass that is simply called a lot.

The relation scheme is identical in structure to the previous example